1. Field of the Invention
This invention pertains to novel zinc dialkyldithiophosphate compositions which are useful as anti-oxidant and anti-corrosion additives in a lubricant oil composition. In particular, this invention relates to zinc dialkyldithiophosphate compositions containing a high lower alkyl content. Said compositions contain from about 0.5 percent to 10 percent by weight of an alkyl or alkenyl mono- or bis-succinimide. The addition of the alkyl or alkenyl mono- or bis-succinimide substantially reduces the susceptibility of the zinc dialkyldithiophosphate compositions of this invention to form a precipitate, thus improving their solution stability.
2. Description of the Prior Art
Zinc salts of dialkyldithiophosphoric acids are known to inhibit the oxidation of the lubricant oil while improving the anti-corrosion property of the lubricant oil composition. It is also known that the alkyl groups of these zinc dialkyldithiophosphates may be of high molecular weight or low molecular weight. Zinc dialkyldithiophosphates wherein the alkyl groups are of four carbon atoms or less are considerably less expensive than the zinc dialkyldithiophosphates containing alkyl groups derived from alcohols having five or more carbon atoms. In spite of this, these zinc dialkyldithiophosphates containing only alkyl groups derived from alcohols having four or fewer carbon atoms have not been widely marketed as lubricating oil additives. Reasons for this have been that the zinc di(loweralkyl)dithiophosphates are insufficiently oil soluble to allow preparation of a concentrate. Moreover, the normally liquid compositions of the low molecular weight zinc dialkyldithiophosphates are prone to precipitation (crystallization).
Normally, the zinc dialkyldithiophosphate additives of this invention are oils and are transported either neat, that is only as the zinc dialkyldithiophosphate, or as a lubricating oil concentrate. In any event, precipitation of a portion of the additive during transport requires that the precipitate be solubilized prior to formulation.
In order to overcome this problem, U.S. Pat. Nos. 2,680,123; 3,151,075; 3,000,822; 3,385,791 and others teach the use of zinc dialkyldithiophosphates prepared from a mixture of a low molecular weight alcohol and a high molecular alcohol. This results in mixture of products containing a statistical distribution of pure zinc di(loweralkyl)dithiophosphate, zinc mixed-dialkyldithiophosphates and pure zinc di(higheralkyl)dithiophosphate as shown in the formula below: ##STR1## wherein R may be either a lower or higher alkyl group.
The use of the mixed alcohols improves the oil solubility of the resulting product while also lowering its overall costs.
However, as the percent by mole ratio of the lower alcohol to the total alcohol employed in preparing the mixed dialkyldithiophosphate exceeds approximately 50 percent, problems with the solution stability of the resulting zinc dialkyldithiophosphate composition are again raised. This problem results in a precipitate forming in the zinc dialkyldithiophosphate solutions. This problem of precipitation in zinc dialkyldithiophosphates of high lower alkyl content is most apparent when the lower alkyl methyl, ethyl, n-propyl and isopropyl groups are employed but also arises when isomers of butyl alcohol are employed. This is particularly evident when the amount of butyl alcohol used to form the zinc dialkyldithiophosphate is raised to approximately 75 to 90 percent of the total alcohol content.
Without being limited to any particular theory, it is believed that, as the level of the lower alcohol employed to form the zinc dialkyldithiophosphate is raised, the statistical distribution of products favors greater amounts of pure zinc di(loweralkyl)dithiophosphate (wherein all the R groups in FIG. 1 are the same lower alkyl group). These pure zinc di(loweralkyl)dithiophosphates are known to be prone to crystallization in solution and to be both insoluble in oil. Thus, although the use of a lower alcohol is economically favorable, the use of increasing amounts of the lower alcohol results in solubility and crystallinity problems in the product.
Accordingly, the use of a mixture of lower and higher alcohols in preparing zinc dialkyldithiophosphates is limited to mole ratios of lower to higher alcohols which favor a low statistical proportion of pure zinc di(loweralkyl)dithiophosphate. Nevertheless, the use of a maximum amount of the lower alcohol is economically advantageous and thus a solution which alleviates the problems of precipitation and solubility associated with the use of large amounts of the lower alcohol would be particularly beneficial.
This invention is directed to zinc dialkyldithiophosphate compositions of high lower alkyl content which are not prone to precipitation under normal storage conditions.
Yamaguchi, in U.S. Pat. No. 4,306,984 as well as in U.S. Ser. No. 437,371, discloses that oil insoluble salts of diloweralkyldithiophosphoric acid may be solubilized by complexing these salts with from 3:1 to 10:1 weight ratio of an alkenyl or alkyl mono- or bis-succinimide.
Likewise, Yamaguchi et al., in U.S. Pat. No. 4,443,360 as well as in pending U.S. Ser. No. 443,362, disclose that oil insoluble salts of cycloalkyldithiophosphoric acid may be solubilized by complexing these salts with from 3:1 to 25:1 weight ratio of an alkenyl or alkyl mono- or bis-succinimide.
I have now found that precipitation (crystallization) of zinc dialkyldithiophosphate compositions prepared from the reaction of phosphorus pentasulfide with a mixture of about 50 to 90 mole percent of a lower alcohol plus 50 to 10 mole percent of a higher alcohol followed by the neutralization of the resulting dialkyldithiophosphoric acid with a basically reacting zinc compound may be alleviated by the addition to the composition of about from 0.5 to about 10.0 percent by weight of an alkyl or alkenyl mono- or bis-succinimide. This is particularly surprising in view of the large excess of succinimide required by Yamaguchi and Yamaguchi et al. to prepare an oil-soluble zinc dithiophosphate complex.
This surprising result of this invention is particularly advantageous as the smaller amount of succinimide required in the zinc dialkyldithiophosphate composition results in concentrates of less bulk and, accordingly, easier and cheaper transportation.